The present invention is directed to broaching, especially as used for machining finished slots with a single pass, and particularly to a novel broaching tool.
Broaching tools are used for machining metal, such as for forming slots in workpieces like turbine wheels. One such broaching tool is disclosed in Clappe et al U.S. Pat. No. 4,243,347 and another is disclosed in Applicant's copending application Ser. No. 392,260 filed Aug. 10, 1989 U.S. Pat. No. 4,993,889.
The broaching tool comprises a holder and a row of broaching inserts or cutters mounted in the holder. Each insert includes a mounting portion seated within a cavity of a holder, and a cutting portion projecting above the cavity.
During a broaching operation, each slot is cut progressively by sequentially contacting the workpiece with the broaching inserts, i.e., initially with inserts which rough-out the slot, and then with inserts which complete the slot. In particular, the workpiece is sequentially contacted with three different types of inserts, i.e., initially with a series of slotters, then with a series of semi-finishers, and finally with a series of finishers, each series of inserts being mounted in a respective holder. The holders are positioned in end-to-end fashion such that the workpiece is contacted successively by a single pass of the inserts.
The slotters gradually rough-out a slot to a given depth by sequentially removing sections of the slot. The semi-finishers then enter the roughed-out slot and gradually widen the sides of the slot to establish the size and configuration of the slot to a degree approximating the desired final size and shape. The finishers function to remove the last film of material needed to obtain a slot within the desired narrow manufacturing tolerances. In a typical broaching assembly, approximately 60% of the cutting inserts are slotters, 30% are semi-finishers and 10% are finishers.
It is desirable that the finished slots be smooth and precisely configured as possible. It is also desirable that the cutting inserts be long lived, due to the considerable time and expense involved in sharpening and/or replacing the numerous inserts employed in a typical broaching assembly.
During a broaching operation, numerous factors may adversely affect insert behavior in a manner making those goals difficult to achieve. One factor relates to the entry of chips or shavings into a space ahead of each insert. In that regard, it will be appreciated that the chips cut from the workpiece by a given insert will be received in a relatively confined space defined by the front end of the given insert, the rear end of the preceding insert, and the wall of the slot being cut. It is desirable that the space be large enough to permit the chip to curl. Otherwise, the chips may tend to become jammed within the space, producing in the need for the insert to re-cut the chip. This can result in premature chippage and breakage of the cutting edge.
One way of providing sufficient chip space is to reduce the insert thickness in the direction of tool travel, i.e., to terminate the rear end of the cutting portion of each insert short of the front end of the next insert to form a gap therebetween. However, this reduction in the insert thickness weakens the insert and may result in premature failure thereof.
Another way of providing chip space would be to shape the rear end of each insert as a narrow web which extends into contact with the front end of the trailing insert. However, only a very limited amount of chip space can be created by such an expedient.
Some broaching holders are characterized by the provision of individual pockets into which the inserts can be mounted. A portion of the holder located behind each insert would provide reinforcement. However, the number and size of inserts which can be inserted into such a holder is restricted by the number and size of the pockets, in contrast to holders of the type wherein the inserts are mounted in a single longitudinally elongated cavity.